Sigma-delta modulator circuits may digitize an analog input signal. These circuits often use a 1-bit quantizer, noise shaping and a high oversampling rate in order to digitally render a signal without requiring the sharp anti-aliasing filters that are needed for standard sample-and-hold digital-to-analog converters.
An exemplary first order sigma-delta modulator is shown in FIG. 1 and includes an integrator 100 that receives in an input signal x(t) through an input capacitor (not shown). The integrator 100 is generally followed by a 1-bit quantizer 110 and the sigma-delta modulator also includes a feedback loop 120 that feeds back the digitized output signal from the sigma-delta modulator using a digital to analog conversion 140 to the input of the integrator. The input to the integrator 130 is a summing node that sums the difference between the input signal and the analog version of the output value. The loop integrates the error between the output and the input signals, may low pass filter the input signal and high pass filters the quantization noise, which is the difference between the quantizer input and output. The digital output of the sigma-delta modulator is supplied to a digital decimation filter 150 to account for the oversampling.
In a standard switched capacitor sigma-delta modulator charge is provide once each clock period. During a first phase, the input signal is sampled and during a second phase the voltage on the input capacitor is passed to the integrator. In order to increase the speed, sigma-delta modulators may include cross-coupled summing junctions wherein the input to the integrator receives charge in each clock phase. In addition to increasing the speed of the circuit, by allowing charge to be provided to the integration capacitor on each phase, the input capacitors located prior to the integrator may be half the size of a single phase integrator for the same kT/c noise performance.
Sigma-delta modulators are often used in audio circuits and further are used in cellular telephones where power consumption is always a concern.